June, 1939 The Queensland Naturalist 
39 
failed to produce any evidence of exploitation of this 
source. The legume, however, by entering into partner- 
ship with bacteria, which have the faculty of using free 
nitrogen, has solved this problem. The establishment of 
this fact in 1888 by Hellriegel and Wilfarth, and the 
isolation of the bacteria by Beyerinck in 1888, and by 
Prazmowski in 1890, is one of the most important of 
modern developments in agricultural science. It led to 
the discovery that there were a number of races of the 
root nodule bacterium, each attacking its own group of 
leguminous hosts, and incidentally benefiting them by 
fixing nitrogen. These races are specialised in their 
relationships with the legumes, so that it is possible to 
have legumes flourishing in a district, and well provided 
with nodules, so that the soil is well inoculated; and yet 
an introduced legume may fail because of absence of the 
race beneficial to it. This has been the experience with 
lucerne in many countries. The failure of that plant in 
parts of the British Isles was formerly thought to be due 
to climatic factors, but since the development of the 
technique of inoculating seed with the right strain of 
bacteria at the time of planting, success has been attained 
in districts where it was once thought lucerne would not 
grow. We have had similar experience with lucerne and 
clovers in many districts in Australia, where success has 
followed the introduction of the appropriate race of 
nitrogen-fixing bacteria. This need for a specialised race 
of bacterium is no doubt the explanation of the very slow 
spread of lucerne in cultivation from the Russian steppes 
where it is native, in spite of the fact that it has always 
been held in very high esteem as a fodder plant. 
The nitrogen-fixing bacteria of the root nodules are 
usually not looked on as disease organisms, but that is 
their primary nature. The fact that they fix nitrogen, 
which the plant is able to take over from them, is their 
saving grace, but does not alter the fact of their parisit- 
ism. There are non-beneficial strains of the bacterium 
which cause the characteristic root swellings, but give no 
counter-balancing benefit. Thus unthrifty clover may 
have a, well-developed nodule supply, and it is only by very 
careful investigation that the fact is established that the 
plant has entered into an unprofitable symbiosis. 
Root nodules are characteristic of iegume roots, but 
by no means confined to them. Bacteria of the Pseudo- 
monas radicicola type have been found in nodules of 
Alnus, Eleagnus , Myrica, Podocarpus, and Casuarina. 
